An image forming apparatus such as an ink jet printer ejects ink from the nozzle provided on the ink jet head to form an image. The ink may be coagulated or caked when viscosity thereof is increased because of evaporation of the liquid component and the volatile component of the ink. If ink is caked within the nozzle, it may course failure of printing.
To restrain the failure of printing with the caked ink, it is well known, for example, that pressure applied to the ink in the ink jet head is varied to flow ink nearby the nozzle. The ink is stirred with the flow of ink and thus the coagulation of the ink is restrained.